A fuel cell stack is an energy conversion system that causes an electrochemical reaction by supplying a fuel gas and an oxidation gas to a membrane electrode assembly, and thereby converts chemical energy into electric energy. Among other things, a solid polymer electrolyte fuel cell stack using a solid polymer membrane as electrolyte is low-cost and can be easily made compact, and also has a high power density, so that the application for an on-vehicle power supply is expected.
AC impedance of a fuel cell stack is used as one of indicators for optimally controlling operational states of a fuel cell stack. A value of the AC impedance is in a correlation with a wet condition of an electrolyte membrane, and measurement of the value of the AC impedance can detect the wet condition of the electrolyte membrane. When the electrolyte membrane has excessive or deficient moisture, an output power of the fuel cell stack is lowered due to the flooding phenomenon or the dryout phenomenon, and to optimally control operation of the battery cell, it is necessary to maintain the wet condition of the electrolyte membrane at optimal conditions. Japanese Patent Laid-Open No. 2005-332702 proposed a method that an AC signal be applied to a fuel cell stack by using a DC/DC converter connected to the fuel cell stack, and a frequency of the AC signal is varied to detect a voltage response, and thereby AC impedance is measured.    [Patent Document 1] Japanese Patent Laid-Open No. 2005-332702